Side-scan sonar data collected in 1977 and 1983 from the Bristol Channel are compared with 1978 SEASAT images, in order to determine the limiting physical dimensions of positive and negative topographic features detectable by SEASAT. The identification of sea-bed topography is related to a change in current speed which, in turn, is controlled bythe relief:water-depth ratio. In a water depth of 45mwith a surface current speed of about 70cms−1, for example, features > 11 m in relief and with a spacing > 500 m are visible.